1. Field of the Invention
The present invention relates to an optical line terminal, and more particularly, to an optical line terminal which controls one or more optical network units connected thereto.
2. Description of the Related Art
Today""s telecommunications market needs call for rapid deployment of optical access networks, notably the xe2x80x9cFiber to the Homexe2x80x9d (FTTH) systems, to provide more sophisticated data communications services. In particular, ATM-based passive optical networks (ATM-PON) are of great interest as an enabling technology for high-bandwidth low-cost data network systems.
FIG. 23 illustrates a typical ATM-PON system, in which an optical line terminal (OLT) 5 is linked with a plurality of optical network units 1-1 to 1-3 via fiber optic cables 2-1, 2-2, 2-3, and 4 being joined together with a passive optical star coupler 3. ITU-T Recommendation G.983.1 describes how to control such a plurality of ONUs from a single termination circuit of the OLT. This ITU-T recommendation defines dedicated Physical Layer Operation and Management (PLOAM) cells to allow the OLT to monitor and control its subordinate ONUs. Using PLOAM cells, the OLT supplies the ONUs with control parameters for them, and the ONUs send their information back to the OLT. There are a plurality of predefined messages encoded to represent various kinds of information to be sent by the OLT and ONUs.
Each ONU needs permission from the OLT when it transmits PLOAM cells to the OLT. This permission is known as the xe2x80x9cPLOAM grants,xe2x80x9d which are mapped in a PLOAM cell sent from OLT to ONUs (this direction is referred to hereafter as xe2x80x9cdownstreamxe2x80x9d). Each single PLOAM grant permits one specific ONU to send a single PLOAM cell to the OLT (this direction is referred to hereafter as xe2x80x9cupstreamxe2x80x9d). The PLOAM grants have unique pre-assigned values, which vary from ONU to ONU. This nature allows the OLT to control the traffic of upstream PLOAM cells sent from ONUs on the same ATM-PON network, on an individual basis by manipulating PLOAM grant values appropriately.
FIG. 24 shows, in a simplified way, how OLT and ONUS transmit PLOAM cells. It is assumed that there are a plurality (n) of ONUs (ONU #1 to ONU #n) being linked to their local OLT. Small boxes PG1 to PGn represent PLOAM grants corresponding to ONU #1 to ONU #n, respectively. As seen from FIG. 24, the OLT sends downstream PLOAM cells to issue a sequence of PLOAM grants PG1 to PGn. After a predetermined period, the ONUs respond to them by sending upstream messages in the order of ONU #1, ONU #2, . . . ONU #n. When the responding ONU has no information to return, it replies so by sending a PLOAM cell with a what is known as xe2x80x9cno messagexe2x80x9d code. In this way, the ONUs return a PLOAM cell of some kind in response to each PLOAM grant even if they have no substantial information to send.
Downstream messages from the OLT may be divided into the following three categories in terms of how frequently they are sent: (1) messages to be sent regularly (hereafter xe2x80x9ccyclic messagesxe2x80x9d), (2) messages that should be delivered within a predetermined time limit (hereafter xe2x80x9ctime-critical messagexe2x80x9d), and (3) messages to be sent on demand (hereafter xe2x80x9cnon-critical messagexe2x80x9d). Similarly, upstream messages from ONUs can be categorized into the following three groups: (1) messages to be sent regularly, (2) messages that have to be returned to the OLT in response to some particular messages sent therefrom, and (3) messages to be sent at the discretion of ONUs. While the OLT can send those messages at any time as required, ONUs need to obtain a PLOAM grant in advance of transmission of each message.
In addition to the messages described above, the ATM-PON system transport ATM cells containing user data, which are referred to hereafter as xe2x80x9cdata cells.xe2x80x9d As with the above-noted message cells (i.e., PLOAM cells), ONUs need permission from their local OLT before sending those data cells. This permission is known as xe2x80x9cdata grants.xe2x80x9d Similarly to PLOAM grants, data grants are mapped on a certain field of a PLOAM cell for delivery to ONUs at the beginning of each frame, where different codes of data grants are assigned to individual ONUs. The OLT communicates with ONUs by sending and receiving message cells and data cells, while issuing such PLOAM grants and data grants as required.
In the meantime, the upstream signals from ONUs in an ATM-PON system are transported over separate optical transmission lines initially and combined by an optical star coupler before reaching the OLT. Since the OLT-ONU distances may vary from ONU to ONU, the upstream transmission signals reach the OLT after different delay times. Therefore, without appropriate delay compensation mechanisms, upstream cells sent by the ONUs in response to PLOAM grants or data grants could collide with each other at the point where the signals are combined (i.e., optical star coupler), thus disrupting the data to be received by the OLT.
To work around the problem of uneven signal delays, each ONU employs an equalization mechanism which intentionally adds an appropriate time delay to its outgoing cells. More specifically, this additional delay equals the difference between each ONU""s specific delay time and the maximum delay time observed in the ATM-PON system, so that all ONUs"" round trip delays will be totally equalized. Such ONU-specific delay times are measured when each ONU is activated as a node in the ATM-PON system. When a new ONU is connected, the OLT stops issuing any grants to temporarily clear out the upstream traffic, and instead issues unassigned grants to indicate that a delay measurement process is under way. This time period established for delay measurement is referred to as the xe2x80x9cranging window,xe2x80x9d or simply xe2x80x9cwindowxe2x80x9d in the G.983.1 terminology. When such a window is open, the OLT causes the target ONU, which is now in the process of activation, to return a certain message for delay measurement. Observing a subsequent response from the target ONU, the OLT measures the cell delay time specific to that ONU. The delay time measured in this way will be used to avoid a upstream cell collisions during normal communication operations.
While the specifications of such ATM-PON systems are provided in the ITU-T recommendations G.983.1, the lack of some detailed definitions actually causes several problems in the following areas.
(1) Discarding of Upstream Messages in ONUs
As previously noted, each ONU needs to get permission, or PLOAM grants, from its local OLT, when it transmits a PLOAM cell to the OLT. Upstream messages from ONUs, on the other hand, are not uniform, but include (a) messages to be sent regularly, (b) messages to be sent in response to the OLT""s requests, and (c) messages to be sent at each ONU""s discretion. This means that the OLT has to send PLOAM grants in a timely manner, taking into consideration the frequencies of those upstream messages. If it is unable to do so, the OLT is likely to miss some messages from the ONUs.
One possible method to satisfy the above requirement is the use of buffer storage in an ONU to temporarily hold a certain amount of messages, so that the ONU will transmit all its pending messages without discarding them as long as the OLT""s PLOAM grant delays are within a tolerable range. However, this requires the OLT to issue PLOAM grants that outpaces the production of messages in the ONUs. Otherwise, the buffer in an ONU would be gradually filled with the pending messages, some of which should be discarded in the end. Although some predictable messages such as those of types (a) and (b) might not be discarded as long as the OLT can send PLOAM grants constantly for them, this method cannot work well for unpredictable messages of type (c).
(2) Discarding of Upstream Messages in OLT
In general, OLT has buffer storage for temporarily storing messages (PLOAM cells) received from ONUs for later parsing and processing. Some messages require a long processing time more than one cell interval. Also, other higher-priority tasks may interrupt the message handling process, keeping it in a suspended state for a while. If such conditions continue, the OLT would be forced to discard some pending messages due to the message buffer overflow.
(3) Timing Contentions Among Downstream Messages
Downstream messages can be classified into the following groups: (a) cyclic messages, (b) time-critical messages, and (c) spontaneous, non-critical messages. The OLT must send those messages in a timely manner to satisfy their temporal requirements. Suppose here that there is a spontaneous message of type (c) conflicting with a temporally restricted message of type (a) or (b). If the OLT serves the former message first, for instance, the temporal requirement of the latter messages may not always be ensured.
(4) Allocation of Upstream Bandwidth
The OLT is responsible for allocating necessary upstream bandwidths to ONUs on the network by issuing data grants in an appropriate manner. However, no practical methods have been proposed in this technical area.
(5) Pending Grants When Ranging Window is Open
The OLT opens a ranging window by issuing unassigned grants only when measuring the cell delay time (or distances) of an ONU. This window period occupies time slots for data cells and PLOAM cells, meaning that some data grants and PLOAM grants ONUs should be queued while the window is open. On the other hand, the service provider must guarantee a specific user bandwidth for conformance to the service contract with their customers. In other words, the OLT is obliged to continuously output a prescribed number of data grants per unit time, and for this reason, it is necessary to ensure the issuance of data grants when it is interrupted. To date, however, no practical methods have been proposed to satisfy this requirement. Another related problem is that the issuance of data grants may be postponed until PLOAM grants are processed.
(6) Handling of Queued Message During Window Period
The OLT has a first-in first-out (FIFO) buffer to store incoming messages from ONUs before parsing and processing them. Delay measurement messages received from ONUs during a window period are also entered to the FIFO buffer. The OLT has to test whether those delay measurement messages have been returned properly within the predetermined window period, as well as filtering out other kinds of messages received during that period.
On the other hand, the message handler in the OLT processes incoming messages in the order of reception, reading out each from the FIFO buffer. That is, the processing of upstream messages is performed without reference to the reception time of each upstream cell. This leads to a lack of synchronization between the incoming messages and window period indicator, thus making it difficult for the OLT to test whether each delay measurement message has been returned properly within the window period.
(7) Validity Checking for Delay Measurement Messages
In a ranging process, the OLT measures upstream cell delay times by analyzing the phase of each received delay measurement message. In parallel with the measurement, the OLT has to check whether the received delay measurement message is correct or not. Conventional OLTs, however, are unable to make this test correctly, since they perform the delay measurement and message handling processes asynchronously with each other for the same reason as described in the preceding item (6).
Taking the above into consideration, an object of the present invention is to provide an optical line terminal which can send and receive messages more reliably.
To accomplish the above object, according to the present invention, there is provided a an optical line terminal which controls one or more optical network units connected thereto. This optical line terminal comprises the following elements: a cyclic trigger signal generator which generates a trigger signal at predetermined intervals; a PLOAM grant generator which produces a message transmission permission (or PLOAM grant) which permits one of the optical network units to transmit a message, in synchronization with the trigger signal generated by the cyclic trigger signal generator; and a PLOAM cell transmitter which transmits to the optical network units the message transmission permission produced by the PLOAM grant generator.